Unlike most other electronic products, electronic locks must be exceedingly rugged to withstand severe physical abuse. This prerequisite imposes a lower limit beyond which critical mechanical components can no longer be arbitrarily made smaller. For reliable performance, forces driving these mechanical components necessarily must have large safety margins. Most existing electromechanical locks employ as transducers electromagnetic devices such as electromagnets, solenoids or motors to translate electrical signals into mechanical outputs. For example, U.S. Pat. No. 5,542,274 (Thordmark et al.) discloses a cylinder lock in which an electric motor is used to move a blocking element. U.S. Pat. Nos. 6,000,609 and 6,374,653 B1 (both Gokcebay et al.) disclose an electronic lock using a solenoid to move a blocking pin. In U.S. Pat. No. 5,351,042 (Aston) one of the ways to keep a locking bar from blocking the barrel of the lock from turning is by energizing an electromagnet which in turn keeps the locking bar from dropping into a recess in the barrel.
Besides having the usual drawbacks of high power consumption, susceptibility to vibration and external magnetic field (electromagnet and solenoid), these conventional electromagnetic devices can only be made small to a degree before forces produced by them become too feeble to be useful. Furthermore, manufacturing of ever smaller electromagnetic devices soon becomes prohibitively costly.
Certain metal alloys, such as TiNi, can be deformed at low temperature and then returned to their original shape after heating. This shape memory effect requires that a martensitic phase change to occur, and that the specific volumes of the martensite (the low temperature phase) and austenite (the high temperature phase) in the alloys are effectively equal. When in the martensitic condition, deformation strains can be “stored” through a mechanical twinning process. The austenite phase cannot accommodate these twins, so that when the material in the martensitic condition is heated and reverts to austenite (this occurs from about 70 to 120 degrees centigrade for commercial shape memory metal wire), the deformed material must also return to its original shape. Fine wires made of shape memory metal TiNi and sold by Dynalloy, Inc. of Costa Mesa, Calif., USA have tensile strength equal to that of stainless steel. Heating a TiNi wire stretched under tension can produce very large pull forces, e.g., a wire of 0.012″ in diameter can produce a maximum pull of 1.25 kg! These shape memory metal wires can also be made extremely fine. Off-the-shelf stocks from Dynalloy, Inc. can go as fine as 0.0015″ in diameter. Shape memory metal wire therefore lends itself to miniaturization.
Shape memory metal wire is used in some prior art to activate actuators. In U.S. Pat. No. 5,977,858 (Morgen et al.) two separate shape memory segments are used to move a leaf spring from one to the other steady states, which thereby closes or opens an electrical circuit, or causes a cantilever to close or open an electrical circuit.
Shape memory metal wire is also employed as the electromechanical transducer in electronic locks in some prior art, as in U.S. Pat. No. 6,008,992 (Kawakami) and U.S. Pat. No. 6,310,411 B1 (Viallet). In both patents, shape memory metal wire is used to directly move a locking bolt. Both patents deal with situations in which external electrical power is available to either operate the lock or to recharge the battery that operates the lock; hence, high power consumption is not a problem. If such arrangement is adapted for use in a high traffic, hard-wired door lock, it would take a sizable backup battery to ensure proper performance in an electrical blackout. Worse yet, if they are used in a stand-alone, battery-powered electromechanical lock, battery life would be unacceptably short.
In U.S. Pat. No. 5,351,042 (Aston), a shape memory metal wire is anchored at one end to a tension spring and at the other end to a locking bar. The position of this locking bar either allows or blocks the turning of the plug inside a lock cylinder. There are two problems with this arrangement. First, soldering or welding cannot be used to join the shape memory metal wire to the spring because heat from the process would destroy the shape memory metal wire. If adhesive is used instead, the joint would not hold over many operation cycles. That leaves us with the most common method of joining shape memory metal wire with anchors, namely crimping, riveting, eyelet-setting or screw tightening. At such tiny scale it is extremely difficult, if not impossible, to perform such joining.
Second, even though shape memory metal wire can be stretched as much as 8% at low temperature and subsequently recovers when heated, it would fail to function after a relatively few, e.g. under 100, cycles. For reliable performance over an acceptable number of cycles the stretching of the shape memory metal wire at low temperature must be kept to some low percentages of its total length. In general, 5-6% stretch would produce wire life of tens of thousands of cycles; at 3-4% stretch, hundreds of thousands of cycles and at under 2% stretch, millions of cycles. Since in the Aston invention the tension spring absorbs part of the contraction and force intended for moving the locking bar, it is necessary to substantially increase the contraction and force of the shape memory metal wire to compensate for this absorption. Further increase in contraction and force is needed to compensate for tension spring variations. Such increase in contraction and force makes it necessary to use longer and thicker shape memory metal wire, which takes up more room and consumes more energy.